Method for discriminating between operating conditions in medical pump

ABSTRACT

A method is disclosed for determining the operating condition of a medical pump based on data derived from a pressure sensor and a position sensor. The pressure sensor generates pressure data by sensing the force on the pumping element. The position sensor generates position data by tracking the pumping cycle and determining the position of the pumping element. The pump pressure data and pump position data are processed and the calculated results compared with a pre-determined threshold value to determine the operating condition of the pump. The three main types of operating conditions of concern are the following: normal condition, where liquid is present and no leaks exist in pumping chamber; leak condition, where liquid is present but a leak exists in the pumping chamber; and air stroke condition, where the chamber contains some air.

RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 11/503,471 filed Aug. 11, 2006, which is a divisionalapplication of U.S. patent application Ser. No. 10/624,667 filed Jul.22, 2003 and issued as U.S. Pat. No. 7,104,763 on Sep. 12, 2006, whichclaims the benefit of U.S. provisional application No. 60/418,914 filedOct. 16, 2002 and U.S. provisional application No. 60/418,986 filed Oct.16, 2002, the disclosures of each of which are incorporated by referencefor all purposes.

BACKGROUND OF THE INVENTION

The present invention relates to a method of determining the operatingcondition of a medical pump. More particularly, this invention relatesto a method of determining fluid status in positive displacement fluidpumping devices for the delivery of fluids to a patient.

Modern medical care often involves the use of medical pump devices todeliver fluids and/or fluid medicine to patients. Medical pumps permitthe controlled delivery of fluids to a patient, and such pumps havelargely replaced gravity flow systems, primarily due to the pump's muchgreater accuracy in delivery rates and dosages, and due to thepossibility for flexible yet controlled delivery schedules. Of themodern medical pumps, those incorporating a diaphragm or pump cassetteare often preferred because they provide a more accurate controlled rateand volume than do other types of pumps.

A typical positive displacement pump system includes a pump devicedriver and a disposable cassette. The disposable cassette, which isadapted to be used only for a single patient and for one fluid deliverycycle, is typically a small plastic unit having an inlet and an outletrespectively connected through flexible tubing to the fluid supplycontainer and to the patient receiving the fluid. The cassette includesa pumping chamber, with the flow of fluid through the chamber beingcontrolled by a plunger or piston activated in a controlled manner bythe device driver.

For example, the cassette chamber may have one wall formed by a flexiblediaphragm which is reciprocated by the plunger and the driver to causefluid to flow. The pump driver device includes the plunger or piston forcontrolling the flow of fluid into and out of the pumping chamber in thecassette, and it also includes control mechanisms to assure that thefluid is delivered to the patient at a pre-set rate, in a pre-determinedmanner, and only for a particular pre-selected time or total dosage.

The fluid enters the cassette through an inlet and is forced through anoutlet under pressure. The fluid is delivered to the outlet when thepump plunger forces the membrane into the pumping chamber to displacethe fluid. During the intake stroke the pump plunger draws back, themembrane covering the pumping chamber pulls back from its prior fullydisplaced configuration, and the fluid is then drawn through the openinlet and into the pumping chamber. In a pumping stroke, the pumpplunger forces the membrane back into the pumping chamber to force thefluid contained therein through the outlet. Thus, the fluid flows fromthe cassette in a series of spaced-apart pulses rather than in acontinuous flow.

One of the requirements for a medical pump is that it is able to detectwhen it is operating under certain abnormal situations and to alert theuser to these problems. Specifically, the pump should detect when flowof fluid is blocked, there is no fluid in the line, there is no cassettein the pump, if the pump has primed correctly, and if the valves in thepump are sealing properly.

Previous pumps that could supply all this information used at least twosensors associated with the pump chamber or tubes to provide input tothe control system. The use of multiple sensors requires more physicalspace on the pump and potentially results in a higher unit manufacturingcost.

It is therefore a principal object of this invention to provide methodsof using single pressure sensor to discriminate between operatingconditions in a medical pump.

These and other objects will be apparent to those skilled in the art.

SUMMARY OF THE INVENTION

A method is disclosed for determining the operating condition of amedical pump based on data derived from a pressure sensor and a positionsensor. The pressure sensor generates pressure data is by sensing theforce on the pumping element. The position sensor generates positiondata by tracking the pumping cycle and determining the position of thepumping element. The pump pressure data and pump position data areprocessed. The processed data is compared with a pre-determinedthreshold value to determine the operating condition of the pump. Thethree main types of operating conditions of concern are the following:normal condition, where liquid is present and no leaks exist in pumpingchamber; leak condition, where liquid is present but a leak exists inthe pumping chamber; and air stroke condition, where the chambercontains some air.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing data from a pump cycle illustrating normal,leak and air stroke conditions;

FIG. 2 is an enlarged view of the graph of FIG. 1, taken along line 2-2,showing data from a pump cycle illustrating normal, leak and air strokeconditions;

FIG. 3 is a graph showing data from a pump cycle illustrating normalstroke conditions with various back-pressure levels;

FIG. 4 is a flow chart illustrating one embodiment of determining theoperating condition of a medical pump according to the presentinvention;

FIG. 5 is a flow chart illustrating another embodiment of determiningthe operating condition of a medical pump according to the presentinvention;

FIG. 6 is a flow chart illustrating another embodiment of determiningthe operating condition of a medical pump according to the presentinvention; and

FIG. 7 is a schematic diagram of the cassette pump, illustrating thefunctional components of the pump and the cassette.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The present invention will be described as it applies to its preferredembodiment. It is not intended that the present invention be limited tothe preferred embodiment. It is intended that the invention cover allmodifications and alternatives that may be included within the scope ofthe invention as defined by the claims that follow.

It will be understood by one of ordinary skill in the art that the termmedical pump as used herein includes but is not limited to enteralpumps, parenteral infusion pumps, ambulatory pumps, or any positivedisplacement fluid pumping device for the delivery of fluids to apatient.

FIG. 7 is a schematic diagram illustrating the functional components ofa medical pump 10, which is used in connection with a disposablecassette 12 for delivering a fluid to a patient. The medical pump 10 andcassette 12 are shown with several components for implementing thepresent invention. Those of ordinary skill in the art will appreciatethat the pump 10 and cassette 12 include many more components than thoseshown in FIG. 7. However, it is not necessary that all these componentsbe shown in order to disclose an illustrative embodiment for practicingthe present invention.

Details of pump 10 and cassette 12 that are not discussed below can bedetermined by reference to commonly assigned and co-pendingnon-provisional application entitled MEANS FOR USING SINGLE FORCE SENSORTO SUPPLY ALL NECESSARY INFORMATION FOR DETERMINATION OF STATUS OFMEDICAL PUMP, which claims priority from provisional applications U.S.Ser. No. 60/418,986 and 60/418,914, the disclosure and drawings of whichare hereby specifically incorporated herein by reference in itsentirety. This disclosure describes in detail means of using a singlepressure sensor and a single position sensor to supply all necessaryinformation to determine the status of a medical pump. The disclosuresand drawings of the provisional applications U.S. Ser. No. 60/418,986and 60/418,914 are also specifically incorporated herein by reference intheir entirety. Commonly assigned and co-pending non-provisionalapplication U.S. Ser. No. 29/166,389 entitled PUMP CASSETTE disclosesthe particular cassette 12 described below. Pump cassettes and cassettepumps in general are well known in the art of medical fluid delivery, asevidenced by commonly assigned U.S. Pat. Nos. 4,818,186; 4,842,584; and5,000,664, the entire disclosure and drawings of which are herebyspecifically incorporated herein by reference.

Cassette 12 includes a housing 14 on which is disposed an inlet port 16for accepting the fluid flowing from an IV bag or other fluid container(not shown). Similarly, fluid lines (not shown) couple an outlet port 18on housing 14 to the body of a patient.

A pumping chamber 20 is connected in fluid flow communication betweenthe inlet port 16 and the outlet port 18. The pumping chamber 20operates to meter fluid through the cassette 12.

An inlet valve 22 resides between inlet port 16 and the pumping chamber20. Inlet valve 22 operates to physically open and close the fluidcommunication between inlet port 16 and pumping chamber 20.

Similarly, an outlet valve 24 resides between the pumping chamber 20 andoutlet port 18. Outlet valve 24 operates to physically open and closethe fluid communication between pumping chamber 20 and outlet port 18.The pumping chamber 20, inlet valve 22, and outlet valve 24 are alloperatively associated with the pump 10 to control the flow of fluidthrough the cassette 12.

A processing unit 26 with a testing timer 27 is included in pump 10 andperforms various operations described in greater detail below. Adisplay/input device 28 communicates with the processing unit 26 andallows the user to receive output from processing unit 26 and/or inputinto the processing unit 26. Those of ordinary skill in the art willappreciate that display/input device 28 may be provided as a separatedisplay device and a separate input device.

A memory 30 communicates with the processing unit 26 and stores code anddata necessary for the processing unit 26 to calculate and output theoperating conditions of pump 10. More specifically, the memory 30 storesan algorithm code 32 formed in accordance with the present invention forprocessing data to determine the operating condition of the pump 10.

An electric motor 34 is controlled by processing unit 26 is energized bya power supply (not shown) to serve as a prime mover for rotatablydriving a shaft 36.

A pumping element 38 is operatively associated with the shaft 36. Whenenergized, the pumping element 38 reciprocates back and forth toperiodically down-stroke, causing pumping element 38 to press on pumpingchamber 20, driving fluid through cassette 12. On an up-stroke, pumpingelement 38 releases pressure from pumping chamber 20 and thereby drawingfluid from inlet port 16 into pumping chamber 20.

An inlet control element 40 is operatively associated with the shaft 36.When energized, inlet control element 40 reciprocates back and forth toperiodically down-stroke, causing inlet control element 40 to press oninlet valve 22, closing pumping chamber 20 to fluid influx. On anup-stroke, inlet control element 40 releases pressure from inlet valve22 and thereby allows the flow of fluid from inlet port 16 into pumpingchamber 20.

An outlet control element 42 is operatively associated with the shaft36. When energized, outlet control element 42 reciprocates back andforth to periodically down-stroke, causing outlet control element 42 topress on outlet valve 24, closing pumping chamber 20 to fluid efflux. Onan up-stroke, outlet control element 42 releases pressure from outletvalve 24 and thereby allows the flow of fluid from pumping chamber 20 tooutlet port 18. Thus the open or closed state of pumping chamber 20 iscontrolled by the positioning and movement of inlet and outlet controlelements 40 and 42.

A pressure sensor 44 is operatively associated with the pumping element38. The pressure sensor 44 senses the force on pumping element 38 andgenerates a pressure signal based on this force. The pressure sensor 44communicates with the processing unit 26, sending the pressure signal tothe processing unit 26 for use in determining operating conditions ofpump 10.

One of ordinary skill in the art will appreciate that the pressuresensor 44 may be a force transducer or any other device that canoperatively sense the pressure brought to bear on the pumping chamber 20by pumping element 38.

A position sensor 46 tracks the pumping cycle of pump 10 by determiningthe position of the pumping element 38. The position sensor 46 can beoperatively associated with the shaft 36, a cam or camshaft 76 attachedto the shaft 36, or the pumping element 38 itself. The position sensor46 generates a position signal by directly or indirectly detecting theposition of the pumping element 38. For instance, in one embodiment theposition sensor 46 is a Hall Effect sensor having a magnet (not shown)in relational contact with shaft 36. The rotational position of shaft 36can be monitored to indirectly detecting the position of the pumpingelement 38. The position sensor 46 communicates with the processing unit26, sending the position signal to the processing unit 26 for use indetermining operating conditions of pump 10. One of ordinary skill inthe art will appreciate that the position sensor 46 as used hereinincludes but is not limited to mechanical indicators such as pivotingdial indicators, electronic switches, Hall Effect sensors, and opticalbased position detectors.

In operation, at the beginning of a pumping cycle, outlet controlelement 42 operates to close outlet valve 24 so that there is no fluidcommunication between pumping chamber 20 and outlet port 18. Inlet valve22 is opened to permit pumping chamber 20 to be in fluid communicationwith inlet port 16. In the next phase of the pumping cycle, inletcontrol element 40 operates to close inlet valve 22, thereby closingfluid communication between inlet port 16 and pumping chamber 20. Outletvalve 24 continues to remain closed. Next, pumping element 38 begins adown-stroke movement which presses pumping element 38 against pumpingchamber 20, causing pumping chamber 20 to compress, thereby increasingthe pressure within pumping chamber 20. Pressure sensor 44 reads andtransmits this pressure data to processing unit 26. Under normalconditions pumping chamber 20 is compressed sufficiently and a desiredpressure profile is generated. At a given position of shaft 36 or pointin the pumping cycle, the outlet control element 42 operates to openoutlet valve 24 so that fluid flows from pumping chamber 20 to outletport 18. The pump cycle then repeats.

The processing unit 26 retrieves the operating condition algorithm 32from memory 30 and applies it to the pressure and position data receivedfrom this pump cycle. The pump pressure data and pump position data areprocessed. The processed data is compared with a pre-determinedthreshold value to determine the operating condition of the pump. Thethree main types of operating conditions of concern are the following:normal condition, where liquid is present and no leaks exist in pumpingchamber; leak condition, where liquid is present but a leak exists inthe pumping chamber 20 (including at the inlet valve 22 or outlet valve24); and air stroke condition, where the chamber contains some air. Oncethe operating condition is determined, the processing unit 26 outputsthe operating condition display 28 and/or uses the determined operatingcondition to adjust operation of the pump 10.

One of ordinary skill in the art will understand that the thresholdvalues for any of the algorithms disclosed herein are predeterminedempirically from experimental data, and will vary from pump model topump model.

Referring to FIG. 1, the position sensor 46 is used to trigger a captureevent where pressure sensor 44 data is captured for processing andoperating condition discrimination. FIG. 1 shows time plots of thepressure and position signals taken with a prototype unit in thelaboratory. The position signals are digital in nature and take onvalues near 3 or 0 V. The remaining analog signals that rise and fallmore gradually are the signals that represent the pressure sensor 44measurements. There is one pressure sensor 44 in the system and the fouranalog signals shown represent four different example operatingconditions that have been superimposed onto the same plot. Each will beused to explain the operation of the signal processing algorithms to bedisclosed.

When large amounts of data under various experimental conditions werecollected, certain observations were made immediately. As shown in theexample set of data shown in FIG. 1, the initial time region between−0.4 s and 0 s did not seem to offer opportunities for signaldiscrimination. Furthermore, other regions beyond 0.2 s also did notseem to offer signal differences that corresponded with the operatingconditions of interest. Specifically, in these regions of non-interest,the back-pressure and other elements in the system seemed to dominatethe pressure signal characteristics. In a region of interest, marked byline 2-2, the system is indeed operating with the pumping chamber 20closed such that the pressure sensor 44 is detecting a building pressureduring the pumping stroke. This allows unique conditions under which itmay be possible to discriminate between normal, leaky, and air-filledpumping conditions.

Referring to FIGS. 1 and 2, the data for the region of interest markedby line 2-2 of FIG. 1 is shown with greater detail. Pump cycle data wascollected in the laboratory by subjecting a prototype pump to a widevariety of operating and environmental conditions to analyze the regionof interest more closely. To develop effective and robust algorithms, itwas important to analyze time shifting, bias shifts or offsets, andother variations that could occur. The four digital position signals arenumbered as A and the four pressure signals are numbered B-E. Theexample pressure signals B-E correspond to the three previouslymentioned operating condition types (Normal, Leak, and Air Stroke), andin addition a back-pressure in the system may be present. The numberedcases in the figure are as follows:

B: Normal type, no back-pressure;

C: Normal type, relatively high back-pressure present;

D: Leak type, low back-pressure; and

E: Air Stroke, low back-pressure.

Those of ordinary skill in the art will recognize that the magnitude,timing, and shape of the pressure signals may vary somewhat depending onthe source or location of the leak(s), amount of air, or amount ofback-pressure. For example, there are at least two more cases orcombinations not shown in FIG. 2. These cases are leak type with highback-pressure and air stroke with high back-pressure.

Data for many other condition combinations were collected and analyzed,and the region of interest (shown in FIG. 1 at line 2-2 and in FIG. 2)remained the most viable one. In particular, data captured prior to anext rising edge G of the position sensor 46 proved to be an effectivedata set. This is due to the widely varying and uncorrelated effectsthat the back-pressure in the system that occurs after this rising edgeG has on the pressure signal. Therefore the specific region of interest(at line 2-2) occurs between the second falling edge F of the positionsensor 46 that occurs in the complete pump cycle and a time point beforethe next rising edge G of this position signal.

A number of algorithms were considered and tested prior to thedevelopment of the final preferred set. Among these included a simplethreshold method and a method in which the falling edge of the pressuresignal was analyzed (falling edge method).

The simple threshold method involved comparing the pressure signalagainst a predetermined threshold. However, varying signal offsets inthe system reduced the performance of this method, making this methodineffective in discriminating between the operating conditions.

Referring to FIG. 3, in the falling edge method, the time derivative (orslope) of the data falling within the region of interest (at line 2-2 inFIG. 1) was calculated and compared to a negative threshold. With thisapproach a falling edge, usually typifying a normal stroke, would resultin a time derivative calculation that would exceed the negativethreshold. Air strokes and certain leak conditions often did not containthis falling edge characteristic and would not exceed the set threshold.However, a normal stroke with a significant back-pressure often did nothave this falling edge. This can be seen in FIG. 3, where some normaltype strokes do have the falling edges when the back-pressure levels arelow and some do not when the back-pressure is high. This conditiontherefore made the falling edge method ineffective in discriminatingbetween the operating conditions.

Other approaches and variations in the same general spirit wereconsidered, but only the preferred approaches below will be described indetail in this disclosure. Three main embodiments of the preferredalgorithms were developed and are listed as follows:

Class 1: Delayed Threshold Algorithm;

Class 2: Weighted Integration Algorithm; and

Class 3: Integrated Split Derivative Algorithm.

There are a variety of possible variations on each class of algorithm.These variations include varying the technique of weighting, disablingthe weighting, position of anchor, and sequence order in which data isanalyzed. The Class 1 delayed threshold algorithm is the preferredembodiment. However, the other algorithms to be described can performequally as well under certain conditions. Therefore, all algorithms areequally important and will be discussed in equivalent detail.

Referring to FIG. 4, the overall operation of the Class 1 algorithm 110is shown in flowchart form. The Class 1 algorithm 110 begins at startblock 112. A decision block 114 monitors the pump cycle through positionsignal A to determine when a region of interest is occurring. In thisexample, the region of interest is specified as starting when the secondfalling edge F of the position signal A is detected in each new pumpcycle. When the second falling edge F is detected at decision block 114,the Class 1 algorithm 110 proceeds to block 116. Block 116 starts thetesting timer 27 for a pre-determined test time Td. Then a block 118acquires a plurality of pressure reference values at some pre-determinedsampling rate during a first portion of test time, and once the first Napressure reference values have been acquired a pressure anchor value iscalculated and stored by averaging these pressure reference values. Thisanchor is stored and will be used in later calculations.

Anchoring is a technique used in this and the other algorithms as aprocess that removes the overall offset variation observed in thepressure signal from one pump cycle to the next and between eachphysical pump unit. This process involves averaging a number of theinitial data points in the data set of interest and subtracting thisaveraged valued from all subsequent data points in the set.

A block 120 acquires a pressure data value and then calculates andstores a resultant value by subtracting the anchor value from the datavalue. A buffer is created and maintained for storing the last Nbresultant value samples (or data points). This buffer may be a circularbuffer to improved processing efficiency. A decision block 122 showsjust such a circular buffer, and repeats the steps of acquiring thepressure data value and calculating and storing the resultant valueuntil the pre-determined test time Td has expired. Thus, as each newpressure data value is acquired the buffer is updated, until thepre-determined value of time Td has elapsed. If time Td has elapsed,then the data acquisition is complete and the final processing occurs.

During the final processing, a block 124 calculates a test value byaveraging the resultant values, and compares this test value with apre-determined threshold value to determine the operating condition ofthe pump. Thus, in the last step, the algorithm 110 averages the Nb datapoints in the storage buffer and compares this averaged value to a setof predetermined thresholds to determine the operating condition of thepump (i.e. normal, leak, or air stroke).

Referring to FIG. 5, the overall operation of the Class 2 weightedintegration algorithm 130 is shown in flowchart form. This algorithmbegins the same way as the Class 1 algorithm 130 begins, but thedifference of operation lies in the update buffer and final steps.

The Class 2 algorithm 130 begins at start block 132. A decision block134 monitors the pump cycle through position signal A to determine whena region of interest is occurring. In this example, the region ofinterest is specified as starting when the second falling edge F of theposition signal A is detected in each new pump cycle. When the secondfalling edge F is detected at decision block 134, the Class 2 algorithm130 proceeds to block 136. Block 136 starts the testing timer 27 for apre-determined test time Td. Then a block 138 acquires a plurality ofpressure reference values at some pre-determined sampling rate during afirst portion of test time, and once the first Na pressure referencevalues have been acquired a pressure anchor value is calculated andstored by averaging these pressure reference values. This anchor isstored and will be used in later calculations.

A block 140 makes the core calculations of algorithm 130 during thebuffer update to calculate an integration term. The following equationdescribes the integration term used in block 140:I_(k)−I_(k-1)+(d_(k)−A)W(t).

Where I_(k) represents the integration term, I_(k-1) represents theprior integration term, d_(k) represents the newly acquired pressuredata value, A represents the anchor value, and W(t) represents theweighting value which is a function of the time (or position) at whichthe new pressure data value was acquired. The function W(t) can belinear, polynomial, or any other function of time to allow the emphasisand de-emphasis of various regions in the data set.

The block 140 sets a first prior integration term I_(k-1) of zero whenthe algorithm first begins during each new pump cycle. The block 140acquires a pressure data value d_(k) and then calculates and stores anew integration term I_(k) by subtracting the anchor value A from thedata value d_(k) to obtain a resultant, multiplying the resultant by aweighting value W(t) to obtain a product, and adding the product to theprior integration term I_(k-1).

A decision block 142 repeats the steps of acquiring the pressure datavalue d_(k) and calculating and storing the new integration term I_(k)until the pre-determined test time Td has expired. Thus, as each newpressure data value d_(k) is acquired the new integration term I_(k) isupdated, until the pre-determined value of time Td has elapsed. If timeTd has elapsed, then the data acquisition is complete and the finalprocessing occurs.

During the final processing, a block 144 compares the integration termI_(k) with a pre-determined threshold value to determine the operatingcondition of the pump (i.e. normal, leak, or air stroke).

Referring to FIG. 6, the overall operation of the Class 3 integratedsplit derivative algorithm 150 is shown in flowchart form. The Class 3algorithm 150 begins the same way Class 2 130 begins, but no anchorcalculation is used, and the update buffer and final steps differ.

The Class 3 algorithm 150 begins at start block 152. A decision block154 monitors the pump cycle through position signal A to determine whena region of interest is occurring. In this example, the region ofinterest is specified as starting when the second falling edge F of theposition signal A is detected in each new pump cycle. When the secondfalling edge F is detected at decision block 154, the Class 3 algorithm150 proceeds to block 156. Block 156 starts the testing timer 27 for apre-determined test time Td.

A block 158 makes the core calculations of algorithm 150 during thebuffer update to calculate a figure of merit. The following equationdescribes the figure of merit calculation used in block 158:FOM_(k)=FOM_(K-1)+(d_(k)−d_(k-q))W(t)

Where FOM_(k) represents the figure of merit, FOM_(K-1) represents theprior figure of merit, d_(k) represents the newly acquired pressure datavalue, d_(k-q) represents any other pressure data value in the set, andW(t) represents the weighting value which is a function of the time (orposition) at which the new pressure data value was acquired. Thefunction W(t) can be a linear, polynomial, or any other function of timeto allow the emphasis and de-emphasis of various regions in the dataset.

The block 158 sets a first prior figure of merit FOM_(K-1) of zero whenthe algorithm 150 first begins during each new pump cycle. The block 158acquires the prior pressure data value d_(k-q) and the new pressure datavalue d_(k), where the prior pressure data value d_(k-q) is any datavalue other than the new pressure data value. The block 158 calculatesand stores a new figure of merit FOM_(k) by subtracting the priorpressure data value d_(k-q) from the new pressure data value d_(k) toobtain a resultant, multiplying the resultant by the weighting valueW(t) to obtain a product, and adding the product to the prior figure ofmerit FOM_(K-1).

A decision block 160 repeats the steps of acquiring the new pressuredata value d_(k) and calculating and storing the new figure of meritFOM_(k) until the pre-determined test time Td has expired. Thus, as eachnew pressure data value d_(k) is acquired the new figure of meritFOM_(k) is updated, until the pre-determined value of time Td haselapsed. If time Td has elapsed, then the data acquisition is completeand the final processing occurs.

During the final processing, a block 162 compares the figure of meritFOM_(k) to pre-determined thresholds to determine the operatingcondition of the pump (i.e. normal, leak, or air stroke). For example,in one embodiment one threshold is set at 450 so that if the Figure ofMerit is above 450, the pump interprets this as a normal fluid stroke;below 450, as an air stroke.

Several variations on each class of algorithms are possible which canenhance performance. These variations include varying the trigger event,technique of weighting, disabling the weighting, position of anchor, andthe sequence order in which data is analyzed.

While the trigger event in the preferred embodiment is the secondfalling edge F of the position sensor, the trigger event can be changedto reduce system variation sensitivity as needed. The trigger event maybe, for example, the second rising edge G in the pump cycle shown inFIG. 1. Setting rising edge G as the trigger event may reduce delaybetween the trigger event and data collection in the Class 1 algorithm110, for example. This is important due to the fact that the Class 1algorithm 110 will perform more satisfactorily if the pressure signaldata collected correlate to a certain desired pump position. Since thereis no pump element position or speed sensing available, the timer andpredicted speed is used to estimate the current position. Shortening thedelay between the trigger event and key data collection will reduce theaccumulating effects of speed variations in the pumping motor andestimated position error, therefore increasing the probability that thecollected data corresponds to the desired and anticipated position.

Another variation involves the smoothing of the data set. It is possibleto acquire all data of interest before algorithm calculations begin. Inthis case, the data can be smoothed prior to core calculations. This iseffective when the pressure signal contains undesirable noise.

The anchor location is another variable that can be changed to enhancesystem performance. In Class 1 algorithm 110 and Class 2 algorithm 130embodiments the anchor is calculated by using the first Na data points.Depending on the curvature and nature of the data set, it may beadvantageous to calculate this anchor by using the data points at someother location within the data set. This may accentuate a certainfeature near the new anchor location and increase the discriminationlevel of the algorithm.

Whereas the invention has been shown and described in connection withthe embodiments thereof, it will be understood that many modifications,substitutions, and additions may be made which are within the intendedbroad scope of the following claims. From the foregoing, it can be seenthat the present invention accomplishes at least all of the statedobjectives.

1. A method for determining operating conditions in a medical pumphaving a cassette with a pumping chamber, comprising: monitoring thepump cycle with a position sensor; starting a testing timer for apre-determined test time at a specified portion of the pump cycle;closing the pumping chamber to flow during at least a portion of thespecified portion of the pump cycle; acquiring a plurality of pressurereference values during a first portion of test time from a singlepressure sensor; calculating and storing a pressure anchor value byaveraging the reference values; setting a first prior integration termof zero; acquiring a pressure data value from the pressure sensor;calculating and storing a new integration term by subtracting the anchorvalue from the data value to obtain a resultant, multiplying theresultant by a weighting value to obtain a product, and adding theproduct to the prior integration term; repeating the steps of acquiringthe pressure data value and calculating and storing the new integrationterm until the pre-determined test time has expired; and comparing thenew integration term with a pre-determined threshold value to determinethe operating condition of the pump.